This invention relates to means for the reproduction of visual color television images through the combination of chassis mounted receiver electronics and a projection system, comprised of three cathode ray tubes, an optical system, and a viewing screen; all elements being appropriately supported within, or attached to, a cabinet of appropriate design and construction.
Contemporary color television sets customarily employ large cathode ray tubes, the screens or faceplates of which are fabricated in a manner that results in the display of three primary colors (typically red, green, and blue) via closely adjacent dots or lines of phosphors, commonly known as a matrix. The separation between and size of the dots in the matrix is intended to be sufficiently small so as to escape the notice of the observer. This is not in fact true at close range. In actuality the pattern of the color matrix intrudes upon the attention of the observer and detracts from the picture resolution, particularly after magnification by projection.
The individual color dots necessarily occupy mutually exclusive elements of the screen area. This results in a severe loss of average screen brightness. Although technically color screens produce the three color effect, seen by the eye as full color, through an additive process, in terms of luminous energy it is a subtractive process since the element of the screen that is emitting in one of the three colors cannot, at the same time, emit in the other two colors. The maximum efficiencies of such screens cannot, therefore, exceed 33 percent. However, since the spaces between the typically round dots of color phosphors are usually filled in by a black background material, the actual luminous efficiency of such screens is typically only 17 percent.
Further image brightness and contrast losses arise in standard television picture tubes because the light emitting phosphors are deposited upon the rear, or inner, surface of the tube face. Each phosphor element serves as an approximately Lambertian radiator. Only a small fraction of the emitted light can escape the front face because of a phenomenon describable as critical angle trapping. This results in the larger part of the emitted light being trapped within the tube face where it would ultimately be reflected back upon the phosphors, thereby reducing the contrast of adjacent image areas. To spoil this contrast reducing effect, tube face plates are typically made of dark glass to absorb the stray light from internal reflections. This also improves the picture quality by reducing undesired ambient light reflections. Unavoidably, however, this technique also attenuates the desired image brightness by typically 50 percent. Only about 20 percent of the emitted light is normally recovered because of the combined losses due to trapping and glass attenuation. When these effects are compounded with the efficiency factor resulting from the color dot matrix formation, supra, the typical luminous efficiency of contemporary color television picture tubes rarely exceeds 3.4 percent.
A further disadvantage of contemporary cathode ray tubes is that they have attained their maximum safe size based upon stress considerations. Larger tubes would be subject to the serious danger of catastrophic implosion. Moreover, manufacturing costs of present color tubes are high, based upon the materials, processes, and close tolerances associated with the production of the three-color phosphor dot matrix.
As a result of the public desire for viewing larger television pictures than are possible with present direct viewing cathode ray tubes, attempts have been made to project enlarged images of the tube face upon a screen. Such attempts have met with inferior results due to the attendant enlargement of the color dot matrix which causes severe reduction in resolution and image quality. Moreover, the already gross luminous inefficiency is further reduced incident to the optical enlargement of the image, thereby providing inadequate illumination for comfortable viewing. Efforts to compensate for the losses through the use of higher power levels are not consistent with safety or with current conservation concepts.
Further attempts were made to overcome these disadvantages by the use of three separate cathode ray tubes, each radiating in one of three primary colors. This approach is more attractive because the tubes do not require a color dot matrix and are therefore about six times as efficient. Additionally, single color tubes are much less expensive to manufacture. However, optical problems arise in the execution of this concept. One must either employ a separate optical system for each tube and project three separate images on the screen in perfect registration, or one must combine the optical paths from the three tubes into one optical system through the use of colored beamsplitters. The first alternative approach is costly, bulky, and the necessary image registration is difficult to achieve and maintain. The second alternative is limited to low aperture optical systems incapable of adequate illumination because the strongly diverging beams of large aperture systems cannot be intercepted by reasonably sized, typically 45.degree. angle oriented beamsplitters located between the lens and tubes.
In the invention herein disclosed, the several disadvantages, described above as representative of the prior art, are avoided by the use of novel optical means that permit all of the light, consistent with the aperture (f/ number) of the optical system, emitted as the output of three single color cathode ray tubes, each operating in one of three mutually complementary primary colors, to be combined into one large aperture optical system for projection upon a viewing screen.